The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to magnetic resonance angiography (MRA).
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The present invention will be described in detail with reference to a variant of the well known Fourier transform (FT) imaging technique, which is frequently referred to as “spin-warp”. The spin-warp technique is discussed in an article entitled “Spin-Warp NMR Imaging and Applications to Human Whole-Body Imaging”by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then an echo signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (ΔGy) in the sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed. In a typical 3DFT pulse sequence spatial information is encoded along two orthogonal axes and the phase encodings (ΔGy and ΔGz) are both stepped through values to sample Fourier space, or “k-space” in a prescribed manner.
MR angiography (MRA) is the application of magnetic resonance imaging methods to the depiction of the human vasculature. To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. Excellent diagnostic images may be acquired using contrast-enhanced MRA if the data acquisition is properly timed with the bolus passage.
The non-invasiveness of MRA makes it a valuable screening tool for cardiovascular diseases. Screening typically requires imaging vessels in a large volume. This is particularly true for diseases in the runoff vessels of the lower extremity. The field of view (FOV) in MR imaging is limited by the volume of the B0 field homogeneity and the receiver coil size (typically, the FOV<48 cm on current commercial MR scanners). The anatomic region of interest in the lower extremity, for example, is about 100 cm and this requires several FOVs, or stations, for a complete study. This requires that the patient be repositioned inside the bore of the magnet, the patient be re-landmarked, scout images be acquired and a preparation scan be performed for each FOV. All of these additional steps take time and, therefore, are expensive. When contrast enhanced MRA is performed, the repositioning also necessitates additional contrast injections.
Recently gadolinium-enhanced bolus chase techniques have been reported which overcome this difficulty, K. Y. Ho, T. Leiner, M. H. de Hann, J. M. A. van Engleshoven, “Gadolinium optimized tracking technique: a new MRA technique for imaging the peripheral vascular tree from aorta to the foot using one bolus of gadolinium (abs).” Proc. 5th Meeting of ISMRM, p203, 1997. As described in U.S. Pat. Nos. 5,924,987 and 5,928,148, MRA data is acquired from a large field of view by automatically moving the patient table to a plurality of different locations during the scan and acquiring an image at each station. If data is acquired quickly, the movement of the table may be timed to follow the contrast bolus through the vasculature so that peak contrast is achieved at each station.
As described in U.S. Pat. No. 6,912,415, filed on Nov. 26, 2001 and entitled “Method For Acquiring MRI Data From A Large Field Of View Using Continuous Table Motion”, MRA images over an extended field of view can be acquired while continuously moving the patient table through the scanner. This speeds up the scan and enables 3D image data to be acquired, but even shorter scan times are needed in order to track the contrast bolus through the vasculature and acquire high resolution images.
One method for reducing scan time is to reduce the number of phase encoding views that are acquired to sample k-space. One such method is to partially acquire k-space and then calculate the missing data. Such “partial” Fourier data acquisition usually uses Hermitian conjugate symmetry to replace the missing k-space data. Hermitian conjugate symmetry only works if the image is real. Numerous phase errors are present in MRI data that make the image complex. These phase errors result from phenomena such as B0 inhomogeneity, gradient eddy currents, group delays in the gradient amplifiers and receive electronics, and the spatial variation of surface coil receive B1 fields. To enable Hermitian conjugate replacement to work with a complex image, the replacement of the missing k-space data is accompanied by a phase correction which removes the phase errors from this data. One partial Fourier reconstruction algorithm, called “Homodyne reconstruction”, uses two filters to accomplish the Hermitian conjugate replacement and the phase correction, respectively. A Homodyne high pass filter doubles the amplitude of the acquired k-space data which is conjugate to the missing k-space data prior to the Fourier transform. After the Fourier transform, the imaginary part of the image is discarded to complete the replacement step. The phase correction step is accomplished by a Homodyne low pass filter. This filter creates an image from a small portion of k-space data acquired symmetrically around the center of k-space. The phase of this image is subtracted from the phase of the Homodyne high pass filtered image prior to discarding the imaginary part of the image.
Another technique that is used to shorten scan time is referred to generally as a “parallel imaging technique”. Such “pMRI” techniques use spatial information from arrays of RF detector coils to substitute for the phase encoding which would otherwise have to be obtained in a sequential fashion using field gradients and RF pulses. The use of multiple effective detectors has been shown to multiply imaging speed, without increasing gradient switching rates or RF power deposition. For Fourier pulse sequences that sample a rectilinear trajectory in k-space, parallel imaging techniques invariably reduce the number of phase encoding steps needed to sample k-space and thereby reduce imaging time. The coil sensitivity information is used during the image reconstruction to make up for the loss of spatial information.
Parallel imaging techniques fall into one of two categories. They can fill in the omitted k-space lines prior to Fourier transformation, by constructing a weighted combination of neighboring lines acquired by the different RF detector coils. Or, they can first Fourier transform the undersampled k-space data set to produce an aliased image from each coil, and then unfold the aliased signals by a linear transformation of the superimposed pixel values.
Two such parallel imaging techniques that have recently been developed and applied to in vivo imaging are SENSE (SENSitivity Encoding) and SMASH (simultaneous acquisition of spatial harmonics). Both techniques include the parallel use of a plurality of separate receiving coils, with each coil having a different sensitivity profile. The combination of the separate NMR signals produced by these coils enables a reduction of the acquisition time required for an image (in comparison with conventional Fourier image reconstruction) by a factor which in the most favorable case equals the number of the receiving coils used as explained by Pruessmann et al. in Magnetic Resonance in Medicine Vol. 42, p. 952-962, 1999.